A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Typically, a single die is projected on each target portion. In other words, once all of the layers have been projected onto the substrate, each target portion provides, for a single IC (or other device). In some instances more than one IC (or other device) may be provided per target portion. For example, two, three or more ICs may be provided per target portion. In some instances it may be desired to form an IC (or other device) which is larger than the target portion. This may occur when it is not possible to increase the size of the target portion to be the same as the size of the desired device. Where this is the case, two or more images are provided which abut one another, thereby forming the device. At the point at which the target portions abut one another, the functional parts of the device connect with one another. This is known in the art as stitching.
In some instances, where devices are to be projected which comprise more than one target portion, it may be difficult to determine where the devices should be located on the substrate.